While there are multiple network structures capable of supporting broadband services, an ever increasing percentage of broadband providers are opting for fiber optic network structures to support both present and future bandwidth requirements. Cable television (CATV), High Definition Television (HDTV), Voice Over Internet Protocol (VOIP), and broadband internet are some of the most common applications now being supported by fiber optic networks, in some cases directly to the home, commonly referred to as Fiber To The Home (FTTH) or to a building, by applying the Fiber To The Building (FTTB), or Fiber To The Curb (FTTC) concept. These different types of fiber optic networks, which provides different distances between the optical fiber and the end-users, incorporate a wide variety of product to support and distribute the signal from a central office to an optic node, and ultimately to the subscriber, or end-user.
FIG. 1 is a simplified system overview of a fiber-based broadband network, such as e.g. a fiber-based broadband access network 100 according to the prior art, which is based on a single fiber configuration connecting an Optical line Terminal (OLT) 101, having the purpose of distributing broadband services, to an intermediate node, a splitter 102, which is arranged to transmit data in the downlink via one wavelength λDI and in the uplink via another wavelength λUI, in order to enable the OLT 101 to provide two way communication between the OLT and the client side, in the present case represented by Optical Network Units (ONU's) 103a . . . 103n. Each ONU 103a . . . 103n receive and transmit data by the use of time-division multiplexing (TDM), while the splitter 102 splits the available power equally among its different output port.
In the field of fiber-based broadband access, Wavelength Division Multiplexing Passive Optical Networks (WDM-PON) are often seen as being the next step after current PON systems which are based on TDM multiplexing, such as e.g. 10G PON, Gigabyte-capable Passive Optical Networks (GPON), Ethernet Passive Optical Networks (EPON) systems which are currently being developed. The implementation of cost-effective WDM-PON is however still a critical issue and a number of other issues are still to be solved before WDM-PON systems which are based on standardized technology can compete commercially with presently available network solutions.
A simplified system overview of a WDM PON system according to the prior art is illustrated in FIG. 2, where, in resemblance to FIG. 1, an OLT 201 is interconnected to a plurality of ONU's 203a . . . 203n. For the WDM PON system, however, the splitter has been replaced by a Wavelength Multiplexer/Demultiplexer (W MUX) 202 which is used in the downstream to separate wavelength channels λDI1, λDI2, . . . λDIn destined for the different ONUs 203a . . . 203n. In a corresponding way each ONU 203 a . . . 203 n is using a separate wavelength channel λUI1, λUI2, λUIn for transmission of data upstreams. As an alternative to ONUs, Optical Network Terminals (ONTs) may be used, where the former devices are used for distribution to a plurality of subscribers in a FTTB or FTTC context, or as backhaul technology, while the latter devices are configured to support one single subscriber, e.g. by applying the FTTH concept. In the downlink, each of the different wavelength channels λDI1, λDI2 . . . λDIn are provided via a respective separate transmitter (not shown) provided in the OLT 201. In addition, wavelength channels provided from different ONU's, or ONTs, in the upstream direction may be combined in the WMUX 202 in a corresponding way.
It is also recognized that there exist alternative architectures for WDM-PON systems, which combine the concepts described above. In a hybrid WDM/TDM PON system, a wavelength mux/demux is used as in FIG. 2. In the hybrid WDM/TDM PON system however the drop fiber also has a passive splitter, connecting several ONUs to each downstream wavelength channel where data is multiplexed with TDM on top of the wavelength channel. Similarly, a group of ONUs connected to this downstream channel share a common upstream channel where the data is again multiplexed using a TDM approach, e.g. based on a GPON-like protocol. A further variant of a WDM-PON system is that the wavelength selective element, which is located in the ONU, is configured as a fixed or tunable filter which selects the proper downstream channel. In this case a passive power splitter can be used to connect the drop fibers to the feeder fiber. Consequently, the application of the method and arrangement suggested in this document is thus not limited to the architectures shown in FIG. 1-2, but also other combinations of common optical technologies could be used by those skilled in the art to create further variants to a fiber access system based on WDM.
As already indicated above, WDM PON arrangement require high initial implementation costs. One important reason for this comes from the need of expensive optical components in the transmitters and receivers both at the OLT and at the client side, i.e. at the ONU/ONT.
It is therefore desired to provide equipment suitable for WDM PON systems which can be of limited complexity, such that the investment cost for such an infrastructure is limited.
It is important that the ONU/ONT is colorless, which means that the upstream wavelength can be adjusted to any of the wavelength channels used in the system. Different distribution port in the WMUX determine which wavelength channel that is used at a specific ONU/ONT. One way to realize a colorless ONU/ONT is to use a tunable laser for the transmission of data. Also at the OLT, tunable lasers can be used, where one tunable laser may be dedicated for each wavelength port. Alternatively, an integrated array of wavelength stabilized laser transmitters or any other known technique suitable for use in a multi-wavelength transmitter can be used.
The WMUX is usually implemented by an Arrayed Waveguide Grating (AWG). The function outlined in FIG. 2, is typically of a cyclic type, where port #1 uses wavelength λ1, λN+1, λ2N+1, and so on. In the present case, it is possible to use separate wavelengths for the upstream and downstream. In case the AWG is non-cyclic, dual-fiber schemes may be used instead of single-fiber schemes. As a further alternative, bi-directional transmission on the same, or nearly the same wavelength may be applied.
In PON systems it is common that one group of wavelength channels is used for the downstream and another group is used for the upstream. These two groups are normally referred to as different “bands” where these bands are often separated by a guard band of e.g. 15 nm. Typically the channel spacing within one band is 100 GHz, 50 GHz or 25 GHz. For the case of a cyclic AWG, the channel spacing in different bands is not equal due to material and design parameters and choices.
If the WMUX only has one wavelength for each port, i.e. it is a cyclic, bidirectional transmission on the same wavelength can be used. Other alternative embodiments may exist, such as e.g. a dual-fiber system using two WMUX's.
There are a number of ways to implement a tunable laser, with different advantages and complexity. However, in most of the implementations it is difficult, if not impossible, to obtain continuous tuning of the wavelength across the entire wavelength span of the laser. Instead, many tunable lasers exhibit continuous tuning across a limited range. Further, there are a number of lasing modes which together covers the complete wavelength range of interest. A typical behavior of a plurality of modes is illustrated in FIGS. 3a and 3b, where a dependency of change of wavelengths λ1 . . . λ8, for the respective modes in relation to tuning control is shown.
In optical transport systems having a tunable laser with a temperature controlling functionality the temperature controller is configured to keep the laser at a fixed temperature, which may typically be set to 25° C. In such a system, the initial setting is typically retained throughout the system operation, or until the system wavelength is reconfigured. Such a temperature control does however significantly add to the cost of the tunable laser.
Normally the transport system use temperature control to cope with temperature changes. If tunable lasers could operate without temperature control significant cost savings would be realized. Mainly for economical reasons, access networks would preferably use lasers transmitters without any temperature control mechanism. A tunable laser has distinct laser modes which can be tuned over a specific wavelength range. When the temperature changes, the respective wavelength of each mode will shift as indicated in FIGS. 3a and 3b. To cover the entire wavelength range, a laser control circuit therefore must select a laser mode and control the tuning of the selected laser mode. In the following, the upstream link is considered, i.e. a transmitter of the tunable laser is arranged at an ONU and a corresponding receiver for receiving transmitted data is arranged at the OLT. However, the concept suggested in this document can be applied also to the downstream link, i.e. for a transmitter of a tunable laser which is arranged in the OLT, wherein the corresponding receiver is arranged in an ONU or ONT.
For use in access systems, a tunable laser would normally need to operate without temperature control, applying a way of oparation typically referred to as an uncooled operation. One type of such a tunable laser is the so called MGYSOA tunable laser. In such a system, the laser temperature would normally follow the environment over a temperature span from 0° C. to 70° C. Over the applied temperature range, the wavelength of a specific laser mode will therefore shift as shown in FIG. 3b. 
In order for the tunable laser to maintain on a specific wavelength during a temperature change, a transmitter on one laser mode 300, would track the wavelength with it tuning control until it reaches the lower limit on the tuning control. At this point, the transmitter would switch to the next mode, indicated as mode 301 in FIG. 3b, and adjust the tuning control accordingly.
An example of a mode control and a corresponding tuning control for a wavelength varying over time according to FIG. 4a is illustrated in FIGS. 4b and 4c, respectively. During a mode change, transmission need to be interrupted, due to the un-stable wavelength which is evident in FIG. 4a from the time instance of the change of mode and for the duration of a limited time instance, here indicated as Ts. Due to this interruption, the receiver may need to be resynchronized, which may lead to traffic loss during the mode switch.